The present invention generally relates to electrical connectors and more particularly to a solder ball terminal which facilitates the connection of electrical leads to surface mount ball grid array devices, land grid array devices and board to board connections.
In the electronics equipment industry, it is known to provide leads for connection to contact pads of printed circuit boards and other substrates by soldering the leads thereto. However, as the electronics industry continues to move toward greater miniaturization, the size and spacing between the contact pads has accordingly been reduced. This requires a corresponding reduction in the size and spacing of the leads which are connected to contact pads. While the reduction of the spacing between the leads is advantageous from a space-conservation standpoint, it is important to provide sufficient spacing to prevent inadvertent contact between adjacent leads. It is likewise important to ensure that a connection is made between the leads and the contact pads.
Various prior art connection devices have been developed in order to facilitate the connection of high density pads. U.S. Pat. No. 5,030,144 to Seidler discloses a solder-bearing lead including a rectangular bar having a pair of prongs for receiving a solder mass therebetween. The bar is bent at a 90xc2x0 angle so that the portion holding the solder mass is parallel to the mounting surface, and the opposite portion extends perpendicularly from the mounting surface. While this device may be effective in connecting terminals to the mounting surface, it has several drawbacks. For instance, due to the offset design, the alignment of the connector is complicated and, once the connections are made, the offset connector can be difficult to design around. Furthermore, the offset design, in combination with the irregular shape of the solder mass, introduces the possibility that the mass could be misaligned with the mounting pad, since the parallel portion might not always be precisely parallel and since the solder mass may not fit precisely on or in the mounting pad.
A more precise method of connecting terminals to mounting pads involves a technology in which the mounting pads are arranged as a grid and a number of solder balls are used to connect the terminals to the mounting pads. The advantage of using this technology, which is called a ball grid array or BGA, is that the solder balls are configured to enable a more precise alignment of the terminal with the mounting pad because the initial contact point on the mounting pad is a round point on the surface of the solder ball, rather than a wider or irregularly-shaped portion of the solder mass described above. This more precise alignment capability enables the spacing between the terminals to be greatly minimized as compared to the device described above with respect to the ""144 patent.
Conventional solder ball terminals are formed by elevating the temperature of a solder ball to just below the liquid state to cause the solder ball 524 to adhere to the end 522 of a metal terminal 510 (FIG. 8, prior art). Although generally effective, there are several disadvantages to conventional solder ball terminals. For example, the terminal can lack good mechanical security (and hence electrical continuity) since the solder ball can be prone to breaking off from the smooth, flat surface of the terminal. Also, the shape (i.e. roundness) of the solder ball can be affected during the attachment of the ball to the terminal which, in turn, can adversely affect attachment to the mounting pad.
Proper alignment of the solder ball to the terminal can likewise be compromised during the attachment procedure. Also, because the solder ball is attached to the end of the terminal, the distance between the terminal and the mounting pad prior to the application of heat to the solder ball is greater than the diameter of the solder ball, thus increasing the risk of the terminal becoming misaligned with the mounting pad while the solder ball is being liquefied. In addition, if the solder ball is not in contact with the mounting pad, then when heat is applied, the liquefied solder will flow toward the heat source, in this instance the terminal, instead of following the force of gravity toward the mounting pad. In such a case a proper connection to the pad is not made.
It is therefore an object of the present invention to provide a device which allows for precise alignment and placement of a solder ball in a predictable manner for facilitating the connection of electrical leads to a variety of devices, including surface mount ball grid array devices, land grid array devices, and board to board connections.
The present invention provides a solder ball terminal in which the solder ball is physically retained in a counterbored socket at a connection end of the terminal. The solder ball is configured such that less than half of the diameter of the ball is exposed from the terminal in order to minimize the distance between the end of the terminal and the mounting pad before heat is applied to the solder ball to effect the connection between the terminal and the mounting pad.
According to one embodiment of the invention, a solder ball terminal is disclosed which includes a body portion, a connection socket disposed at one end of the body portion, the socket including an interior wall terminating in an annular lip, wherein the annular lip defines an opening into the socket. The solder ball is disposed within the socket such that a portion of the solder ball which is less than half of the diameter of the solder ball, extends beyond the annular lip. In a preferred embodiment, the socket has a spherical shape, and a longitudinal axis of the terminal""s body portion extends through a center point of the solder ball.
According to another embodiment of the present invention, a solder ball terminal for particular application in surface mount circuit board connections is disclosed. The terminal includes a body portion at a first end thereof and a head at a second end thereof, the head including a socket having an opening for receiving a solder ball such that a portion of the solder ball extends outwardly from the socket, beyond the opening. The body portion may also preferably include a male adaptor for connection to a second solder ball terminal.